<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">按钮</button>
</body>
</html>
<script>
    // 异步操作：各自做各自的事，互相不影响


    btn.onclick=function(){
        console.log(1);
    }
    console.log(2);


    setTimeout(function(){
        console.log(3);
    },100)
    console.log(4);





</script>